Ventilating systems for enclosed spaces in which a heat exchanger is placed between the supply air stream and the exhaust air stream to take advantage of the temperature difference, for the purpose of conserving the energy required for either heating or cooling, are known. For example see U.S. Pat. Nos. 1,725,906; 2,206,858; and 2,019,351. Carrying the concept a step further, it is even known that heat pipes may be used in such heat exchange systems as described and illustrated in U.S. Pat. No. 3,788,388.
Such known systems, however present various problems. For example, most of these systems include a plurality of straight, individually sealed and charged pipes which carry a working fluid. Such a system is expensive and complicated. The problems are even further magnified if it is desired to control the rate of heat recovery in response to the temperature of the ventilated space. This is known as "modulating". Heat pipe devices normally transfer heat at a rate solely proportional to the temperature difference between warm and cold ends. Modulating this heat transfer rate has required complicated apparatus.
Further, when the supply air reaches a temperature below a prescribed point, the outgoing air stream will reach a temperature where moisture will precipitate and condense on the pipes. If the surface temperature of the pipes is below freezing, a forming and buildup of frost thereon will occur with undesirable results.